Police used rubber bullets to disperse a crowd of fans after Colorado State University upset the University of Colorado in the schools' annual football showdown.
Law enforcement shut down part of downtown Fort Collins on Sunday after Colorado State's 23-17 win to let students celebrate. Officers then put on riot gear and fired rubber bullets to break up the quickly growing crowd after revelers ignored orders to disperse.
Police were dodging beer cans and bottles.
Officer Kevin Cronin said police were worried that two groups in the area would merge into one big crowd.
"Everyone is text messaging. Eventually the two (crowds) will meet up," Cronin said.
In Boulder, police said they were busy during and after the game on the CU campus. About 75 people were ejected from the game and 15 others were taken to an alcohol detoxification center.
Sgt. Pat Wyton said officers dealt with "the usual drunks and idiots," starting early in the afternoon when police broke up rowdy parties on University Hill across from the Boulder campus before the game started.
Patrol officers also responded to a pre-game report of a portable toilet on fire downtown and reports of women running topless "from party to party" on University Hill.
Boulder police said they issued mainly alcohol-related citations.
Colorado State's win was its first in Boulder since 1986. The intrastate series began 116 years ago, with some interruptions through the years.